1. Field of the Invention
The present invention relates to a nitride crystal, a nitride crystal substrate, an epilayer-containing nitride crystal substrate, a semiconductor device and a method of manufacturing the same, and particularly to a nitride crystal that can be preferably used as a substrate for growing an epitaxial crystal when producing a semiconductor device.
2. Description of the Background Art
As is well known, various devices using nitride semiconductor crystals have been produced in recent years, and nitride semiconductor light emitting devices have been produced as a typical example of such semiconductor devices.
Generally, in a process of manufacturing a nitride semiconductor device, a plurality of nitride semiconductor layers are epitaxially grown on a substrate. Crystal quality of the epitaxially grown nitride semiconductor layer is affected by a state of a surface layer of the substrate used for the epitaxial growth, and this quality affects performance of the semiconductor device including the nitride semiconductor layer. Therefore, in the case where the nitride semiconductor crystal is used as the above kind of substrate, it is desired that at least a main surface of the substrate providing a base of epitaxial growth has a smooth form without a distortion.
More specifically, the main surface of the nitride semiconductor substrate used for the epitaxial growth is generally subjected to smoothing processing and distortion removal processing. Among various compound semiconductors, gallium-nitride-based semiconductors are relatively hard so that the surface smoothing processing thereof is not easy, and the distortion removal processing after the smoothing processing is not easy.
Japanese Patent Laying-Open No. 2004-311575 has disclosed a polishing method that uses soft grains and hard grains as polishing compounds for polishing a surface of a nitride gallium substrate. U.S. Pat. No. 6,596,079 has disclosed a method of forming a substrate surface in the case where the substrate is produced from an (AlGaIn)N bulk crystal grown by vapor phase epitaxy on an (AlGaIn)N seed crystal, and more specifically a method of forming a substrate surface that has a RMS (Root Mean Square) surface roughness of 1 nm or lower, and does not have a surface damage owing to effecting CMP (Chemical-Mechanical Polishing) or etching on the substrate surface subjected to mechanical polishing. U.S. Pat. No. 6,488,767 has disclosed an AlxGayInzN (0<y≦1, x+y+z=1) substrate having an RMS surface roughness of 0.15 nm attained by the CMP processing. A processing agent for this CMP contains Al2O3 grains, SiO2 grains, pH controlling agent and oxidizer. According to Japanese Patent Laying-Open No. 2001-322899, a work-affected layer is removed by dry etching to finish the substrate surface after polishing the GaN substrate.
In the prior art, as described above, the CMP processing or dry etching is effected after mechanically polishing the GaN crystal so that the work-affected layer formed by the mechanical polishing is removed, and the GaN substrate having the finished substrate surface is formed. However, the processing rate of the CMP processing is low, and causes problems in cost and productivity. Further, the dry etching causes a problem in surface roughness.
The finishing method of the Si substrate using the CMP as well as the polishing agent for the method are not suitable for the hard nitride semiconductor substrate, and lower the removal speed of the surface layer. In particular, GaN is chemically suitable, and is relatively resistant to the wet etching so that the CMP processing is not easy. Although the dry etching can remove the nitride semiconductor surface, it does not have an effect of flattening the surface in a horizontal direction so that the surface smoothing effect cannot be achieved.
For epitaxially growing the compound semiconductor layer of good crystal quality on the substrate surface, it is necessary to use the substrate surface having good crystal quality as well as less work damage and less distortion as described above.
However, the crystal quality of the surface layer that is required at the substrate surface is not clear.
In the prior art, distortions at the surface layer of the crystal are evaluated by cleaving the crystal, and observing the cleavage plane with a TEM (Transmission Electron Microscope) as disclosed, e.g., in S. S. Park et al, “Free-Standing GaN Substrate by Hydride Vapor Phase Epitaxy”, Jpn, J. Appl. Phys., The Japan Society of Applied Physics, Vol. 39, November 2000, pp. L1141-L1142 and Yutaka TAKAHASHI et al, “Transmission Electron Microscopy of Surface Damages Resulting from Wet Polishing in a Polycrystalline Aluminum Nitride Substrate”, The Academic Journal of the Ceramic Society of Japan, The Ceramic Society of Japan, 99, [7], (1991), pp. 613-619. Thus, the distortions at the surface layer of the crystal have conventionally been evaluated by a breaking test that breaks the crystal, and therefore such problems occur that correction cannot be performed after the evaluation even when the result of evaluation was insufficient, and that the evaluation cannot be effected on the product in itself. Under the present circumstances, there is no index for nondestructively evaluating the crystallinity of the surface layer at the finished substrate surface, and it is difficult to define quantitatively the crystal quality of the surface layer.